The following relates to the nuclear reactor arts, electrical power generation arts, nuclear reactor control arts, nuclear electrical power generation control arts, and related arts.
Nuclear reactors employ a reactor core comprising a critical mass of fissile material, such as a material containing uranium oxide (UO2) that is enriched in the fissile 235U isotope. The fuel rod may take various structural configurations, for example including fissile material as pellets embedded in a ceramic matrix or so forth. To promote safety, it is conventional to assemble the core as rods containing the fissile material. A set of rods is preassembled to form a fuel assembly. Preferably, the mass of fissile material in the fuel assembly remains below critical mass. The fuel assemblies are shipped to the reactor site, and are installed in a grid in the reactor pressure vessel to form the reactor core. To prevent a premature chain reaction, suitable neutron absorbing material is provided during installation, for example by inserting neutron-absorbing control rods into the fuel assemblies before they are brought together in the pressure vessel, and by omitting the neutron moderator (e.g., water ambient) if employed.
With reference to FIGS. 1 and 2, an illustrative example of such an assembly is shown. FIG. 1 shows an illustrative fuel assembly 10 including a set of fuel rods 12 secured together with a controlled spacing by mid-spacer grid elements 14 and by end-spacer grid elements 16, 18. In the illustrative example, the fuel rods 12 form a 17×17 array. The fuel assembly 10 is typically substantially elongated, and is shown in part in FIG. 1 with an indicated gap G. The fuel assembly 10 also suitably includes other elements, such as control rod guide tubes or thimbles 20 through which neutron-absorbing control rods may pass. One or more of these or similar tubes or thimbles may also serve as instrumentation conduits for in-core sensors. Upper and lower nozzle plates 22, 24 may be provided to facilitate coupling of control rods, instrumentation bundles, or so forth into or out of the fuel assembly 10. The illustrative upper and lower nozzle plates 22, 24 include respective upper and lower alignment pins 26, 28 at the corners of the respective nozzle plates 22, 24 for facilitating alignment of the fuel assemblies during installation in the reactor core.
FIG. 2 shows the assembled reactor core 30, including a closely packed grid of fuel assemblies 10 disposed in a core former 32. In FIG. 2, a control rod assembly (CRA) is fully inserted into each fuel assembly 10. In the view of FIG. 2, only an upper support element 34 of the CRA is visible extending above each corresponding fuel assembly 10. The upper support element of each CRA may in be a conventional spider or (as in FIG. 2) a larger element (see “Terminal Elements for Coupling Connecting Rods and Control Rods in Control Rod Assemblies For a Nuclear Reactor”, U.S. Ser. No. 12/862,124 filed Aug. 24, 2010, which is incorporated herein by reference in its entirety, for some illustrative examples). The illustrative reactor core 30 includes sixty nine (69) fuel assemblies, although in general more or fewer fuel assemblies may be included.
The reactor core has a designed lifetime, typically in a range of a year to a few years. The core lifetime is controlled by the reduction in fissile material caused by operation of the nuclear chain reaction. To continue operation, a refueling operation must be performed, in which the spent fuel assemblies are removed and replaced by new fuel assemblies. Typically, this entails shutting down the reactor, opening the pressure vessel and removing any components in order to gain overhead access to the fuel assemblies, and removing the fuel assemblies with the assistance of a crane. To enable coupling with the fuel assembly, each fuel assembly is typically fitted with a box structure with leaf springs mounted on top of the box, or a plate-and-post structure with preloaded helical coil springs mounted between the posts. The fuel assembly is lifted by a grappling mechanism that engages the fixed top plate of the box structure or the movable top plate of the plate-and-post structure via hooks that swing laterally under the top plate in four orthogonal directions. In box designs, the hooks swing outward to engage the top plate of the box, while in plate-and-post designs the hooks swing inward to engage the top plate.
These refueling approaches have substantial disadvantages. The swinging motion of the grappling hooks calls for a large working space proximate to the top of each fuel assembly. However, this working space is constrained by the presence of closely adjacent neighboring fuel assemblies in the array disposed in the core former. Moreover, if the CRA is left fully inserted into the fuel assembly during refueling (which is desirable to maintain suppression of the neutron population in the fuel assembly during the refueling process), then either the spider must be removed entirely (a process entailing individually detaching each of the numerous control rods from the spider), or the spider must be of sufficiently low profile to enable the grappling hooks to operate above the spider.
Disclosed herein are improvements that provide various benefits that will become apparent to the skilled artisan upon reading the following.